Q.
What is the meningeal worm and do I need to worry about it?
A.
The meningeal worm (Parelaphostrongylus tenuis) is a parasite of white tail deer. As the natural host, deer tolerate it well. As an accidental host, sheep/goats, camelids, and other animals don't tolerate it so well. It is a cause of neurological illness and death. It is a significant problem for some sheep/goat owners.
The meningeal worm is a roundworm with an indirect life cycle. Snails and slugs are the intermediate host. Deer pass eggs in their feces. The eggs/larvae penetrate snails/slugs or are ingested by them. Sheep/goats become infected when they accidentally consume the snail/slug. Once inside the sheep/goat, the larvae penetrate the stomach or intestinal wall and migrate through the body cavity to the central nervous system.
The symptoms of meningeal worm are neurological and vary. Sheep/goats may be lame or stiff, reluctant to move. Posterior paralysis is common. Some sheep/goats show signs of brain disease, such as a head tilt, walking in circles, rapid eye flickering, and difficulty chewing, Once the larvae migrate all over the body, many goats experience intense itching or rubbing. They develop "hot spots." Affected animals often remain bright and have good appetites.
With neurological symptoms, there are many possibilities besides meningeal worm that need to be ruled out, including rabies, scrapie, polio, listeriosis, pregnancy toxemia, milk fever, white muscle disease, tetanus, and copper deficiency. Meningeal worm is difficult to diagnose because there is no live animal test. A fecal sample isn't useful. No eggs are produced. It is even difficult to locate the parasite upon necropsy. Diagnosis is usually made on the basis of clinical symptoms, exposure (grazing) history, and response to treatment.
Sheep/goats grazing wooded areas with high deer density are most at risk for meningeal worm. While the meningeal worm cycle lasts about four months in deer, there is a 45 to 60 day delay in infection to clinical disease in accidental hosts such as sheep/goats. Because snails/slugs hibernate in winter and go dormant in the summer, most disease occurs in the fall and winter. Periods of highest risk can be altered by weather, e.g., moderate summer weather.
Treatment for meningeal worm has several objectives: kill the parasite, reduce inflammation, and support vital functions. There are no FDA-approved treatments. Treatment usually involves high doses of an anthelmintic. Fenbendazole (SafeGuard®) is the current drug of choice. An anti-inflammatory drug is given to reduce inflammation; dexamethasone or flunixin meglumine (Banamine®). All treatment drugs are extra label for sheep/goats and require a veterinary-client-patient relationship. Even with successful treatment, permanent neurological disease is still possible
Though it was pursued at one point, there is no vaccine for meningeal worm. Prevention includes trying to minimize deer populations. Exclusionary fencing and guardian dogs can help. Snails/slugs are another potential control point. Low lying areas should be fenced off. Areas around pastures and barns should be kept clean. Vegetation-free barrier zones can keep snails/slugs from migrating onto pasture. Ivermectin is sometimes given monthly to high-risk animals (usually camelids) to kill the early stages of the parasite before it crosses the blood-brain barrier. Routine use of ivermectin will promote drug resistance among the common parasites, such as barber pole worm.
A few years ago, Cornell University (in New York) conducted research evaluating treatment protocols. Infected animals were given oral fenbendazole for five days and an anti-inflammatory drug (dexamethosone) for 5 days. Half of the animals were given a subcutaneous infection of ivermectin. The other half were given a placebo. The study was somewhat inconclusive as to the benefit of ivermectin. Due to its long withdrawal period (96 days), ivermectin should probably only be used in valuable animals.
6/11/21
Additional reading
Deer worm treatent protocols - Cornell University
Meningeal worm in Central Iowa Goat herds
Paralephostrongylus tenius affects small ruminants